export default {
    bind (el, binding, vnode) {
      // 响应点击后滚动到元素顶部
      el.addEventListener('click', () => {
      const target = binding.arg ? document.getElementById(binding.arg) : window
      target.scrollTo({
        top: 0,
        behavior: 'smooth'
        })
      })
    },
    update (el, binding, vnode) {
      // 滚动到达参数值才出现绑定指令的元素
      const target = binding.arg ? document.getElementById(binding.arg) : window
      if (binding.value) {
        target.addEventListener('scroll', (e) => {
          if (e.srcElement.scrollTop > binding.value) {
            el.style.visibility = 'unset'
          } else {
            el.style.visibility = 'hidden'
          }
        })
      }
      // 判断初始化状态
      if (target.scrollTop < binding.value) {
        el.style.visibility = 'hidden'
      }
    },
    unbind (el) {
      const target = binding.arg ? document.getElementById(binding.arg) : window
      target.removeEventListener('scroll')
      el.removeEventListener('click')
    }
  }
  